The present invention relates generally to multi-mode lasers and more particularly to a multi-mode laser implementation and associated method which provides amplitude-stable, quiet operation.
Various schemes for multi-mode operation of intracavity-doubled lasers have been implemented in the prior art. However, a multi-mode intracavity-doubled laser which exhibits stable amplitude operation has been more difficult to consistently attain. Such amplitude stability is generally expressed as a percentage of root-mean-square (RMS) output beam power. Interest in multi-mode, amplitude-stable lasers arises from applications such as laser spectroscopy and laser printing.
One approach in designing a stable intracavity-doubled laser relies on the presence of a relatively large number of longitudinal modes. As an example, a diode-pumped, multiaxial mode, intracavity doubled laser with high amplitude stability (i.e., less than 3% RMS output beam power) is disclosed in U.S. Pat. No. 5,446,749. In this particular approach, low noise operation is achieved by using a long laser cavity, which yields a multiplicity (50 to 100) of longitudinal modes, thus resulting in an averaging effect whereby to reduce the total noise in the resulting sum average of the output signal.
Another approach attempts to provide stable operation by limiting the number of longitudinal modes present in the laser cavity. One implementation is described by M. Oka and S. Kubota (Opt. Lett. 13, 805, 1988) wherein quarterwave plates are inserted into the laser cavity, thus forcing the laser to operate in orthogonally polarized modes. This method of operation reduces the frequency sum generation process of the fundamental, thus stabilizing the laser output. However, this approach works only with nonlinear media which obeys Type II phase-matching conditions. Moreover, the additional intracavity elements lead to increased losses and a more complex overall structure.
Still, another approach for reducing the number of longitudinal modes is described in U.S. Pat. No. 5,164,947 wherein the laser is forced into single-mode operation by using a twisted-mode technique to eliminate spatial holeburning, thus reducing the output noise. Additionally, two quarterwave plates and a polarizer are positioned within the laser cavity in this approach. At first appearance the single longitudinal mode laser may seem to provide an advantageous method of achieving low noise operation. Unfortunately, however, SLM laser operation is generally attained at the price of much reduced output power. Specifically, unwanted modes are extinguished by the configuration of the laser. In many applications, certain ones of these extinguished modes represent useful energy. The SLM laser also shares at least one disadvantage of Oka and Kubota's approach in that additional intracavity components are required to extinguish the unwanted modes, thus leading to additional intracavity losses and a generally more complex structure.
In view of the foregoing, there remains a need for an amplitude-stable, intracavity frequency multiplied laser that does not require active cavity stabilization or additional intracavity elements, yet produces low noise output in multiple modes, specifically two modes. The present invention provides a heretofore unseen approach and associated method for producing a two mode, amplitude-stable, intracavity-doubled laser which is suitable for use in low noise applications.